This invention relates to a printing head driving apparatus designed especially for use in an impact printer and a thermal printer.
In general, an impact printer is adapted to drive a printing head through a solenoid, effecting a printing operation. As shown in FIG. 1, an impact printer is of such a structure that an impact wire pin 1 is mounted on an armature. By the operation of the armature 2, the impact wire pin 1 is guided toward the forward end of the printing head. The armature 2 is attracted to a core 4, upon excitation of a solenoid coil 3. The printing head is driven in synchronism with the timing of the driving pulse signal (a pulse signal synchronized with a printing timing signal), as shown in FIG. 2a. Thus, an excitation current b flows through the coil 3, shown in FIG. 2b, based on the timing of the driving pulse signal a to excite the solenoid.
When a power source voltage supplied to the impact printer varies, there is the possibility that the printing head will sometimes be driven in an unstable fashion. This is due to the fact that the printing timing, printing pressure, etc., become unstable as a result of variations in the power source voltage and printing quality is thus degraded. Specifically, when comparison is made between the waveform b1 of a driving current (i.e., the excitation current of the solenoid) in the case of a high power source voltage, and a driving current waveform b2, in the case of a low power source voltage, it may be seen that a variation occurs in their rise time, as shown in FIG. 2b. In the driving operation of the printing head, variations in the power source voltage cause the mistiming of the printing operation and/or the lowering of printing pressure energy, due to an attractive force of the solenoid.
In the thermal printer, a printing operation is carried out by a thermal head equipped with a heat generating element. The thermal head is so formed that heat generating elements 5 and a signal line 7 are wire-printed on a substrate 6, as shown in FIG. 3a. The substrate 6 is usually made of an insulating material, such as an alumina ceramic materials and has high thermal conductivity. The heat generating elements 5 are arranged in one row or more on the substrate 6. The number of the heat generating elements 5 is so determined as to correspond to the kinds of dots, such as the number of dots constituting one character and the number of dots for covering the whole surface of a printing paper sheet.
The thermal head is normally made of a thermal element which is formed of a resistor and generates heat upon the expenditure of electrical power. A printing operation is carried out by bringing a paper sheet into direct, firm contact with the heat generating elements 5 shown in FIG. 3b, or by placing a transfer film between the heat generating element 5 and the paper sheet. In this case, the heat generating element 5 generates heat in synchronism with a driving pulse signal, as shown in FIG. 4a. The heat generation timing b of the heat generating element 5 varies, according to variations in the power source voltage of the thermal printer, as shown in FIG. 4b. When comparison is made between a heat generation timing waveform b1, in the case of a high power source voltage, and a heat generation timing waveform b2, in the case of a low power source voltage, their timings are as different in the thermal printer as they are in the impact printer.
To prevent such an inconvenience, a voltage stabilizing circuit is provided in both the conventional impact printer and the thermal printer. Some impact printers have such an arrangement that, for example, a current feedback circuit is provided for each solenoid. In the conventional system, however, a printing head driving circuit, power source circuit, etc., of the impact printer and thermal printer become complicated in their arrangement, increasing the costs of the printer as a whole.